No, the DJI Osmo Pocket 3 is not compatible with the Lexar Professional 2000x 256GB SD.
DJI Osmo Pocket 3 has a microSD-only card slot. Its own product page confirms only microSD card compatibility -- a full-size SD card is physically larger than a microSD card and does not fit this slot (the reverse of the more common microSD-via-adapter case: here there is no adapter that shrinks a full-size card down to fit).
What works instead
See all compatible camera for the DJI Osmo Pocket 3 →“Supported SD Cards microSD (up to 1 TB) Recommended microSD Cards”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)